The invention relates to a method and an arrangement for controlling the operation of a high-frequency power amplifier.
In GSM transmitters, for instance, specifications place rigid restrictions also on the operation of the transmitter as well as on switching on and off the transmission power. At power control and especially at switching off and reswitching the power, strict limit values shall be observed, particularly as far as the range of variations in power levels is concerned. When transmission power is switched off at a GSM transmission according to the specifications, it should be possible to reduce the transmission power during idle time slots between frames by at least 70 dB, in practice however by 80 dB, which can be carried out by decreased gain and by using RF range switching means providing an attenuation of about 40 dB, in addition to the 40 dB obtained by the control of the amplifier. One problem with the previous solutions is that the dynamic range of detector means is only about 40 dB.
In some methods and arrangements according to the prior art, the transmission power is switched off by using a switching means positioned in a power switching loop on the input side of an amplifier, in which case the signal path breaks, however, and the detector output remains floating in a way. This generates a control signal undefined in its details in a control means of the input side of the amplifier, which in turn causes problems, i.e. practically a control error in a situation when the transmission power shall be switched on again. A control error may also cause a decrease in attenuation when the power is switched off. This restricts the permitted maximum gain between the detector and the control means as well, which maximum gain determines in turn the accuracy of the output power in an ON state. Some known solutions are disclosed in European Patent Applications 0 481 524, 0 546 693 and 0 397 444, U.S. Pat. No. 5,128,629 and WO 91/12661.